12 HELEN DEAN KING AND HENRY H. DONALDSON 



Wild gray females living in captivity show a pronounced 

 tendency to destroy their young at birth, as Miller ( '11) has 

 noted. Casualties were so frequent with the litters cast by 

 wild females in our colony that it was evident it would be 

 impossible to obtain the animals needed to start a strain of 

 captive Grays if the mothers were depended upon to rear 

 their offspring. This difficulty was solved by removing the 

 young rats from the nest as soon as they were discovered and 

 giving them to a lactating albino female to rear. In a few 

 cases albino females refused to suckle the substitutes for 

 their own offspring, but in the majority of cases they ac- 

 cepted the young gray rats and reared them successfully. A 

 total of 106 gray rats of the first generation, 51 males and 

 55 females, were brought to maturity and used for breeding 

 stock. 



That the early postnatal growth of gray rats that were 

 suckled by albino foster mothers was not retarded is shown 

 by a test made with a litter cast by a wild female that became 

 relatively tame and did not greatly resent the temporary 

 removal of her young from the nest. Four of the eight young 

 in this litter were given to an albino female to rear ; the other 

 four young remained with the mother. At thirteen days of 

 age the young suckled by the albino female weighed, on the 

 average, 3 grams more than the other members of the litter. 

 At thirty days of age, when all the rats were weaned, the 

 average weight of those reared by the albino female was 

 nearly 4 grams greater than that of the individuals their 

 mother suckled. In later life, individuals of the same sex 

 showed no pronounced differences in body weights. 



Females of the first generation born in captivity did not 

 show such a marked tendency to destroy their young as did 

 their wild mothers. If the nest was disturbed very little 

 when the young were removed for examination, they could be 

 returned with a fair chance for life and future care. Indi- 

 viduals in the second and in all subsequent generations were 

 reared by their own mothers. 



